Electric arc furnaces (“EAFs”) are used in various arts, but are largely used in steel production. When used for steel production, generally EAFs are large, cylindrical structures that operate by using arcs of electricity to heat and melt steel scrap. They often include a melting chamber, graphite electrodes, and a roof apparatus. The melting chamber receives the steel scrap, is enclosed by the roof, and the electrodes are then inserted into the melting chamber through the roof. Generally, EAFs are either single phase direct current (“DC”) systems or three phase alternating current (“AC”) systems (using one electrode for a DC EAF and three electrodes for an AC EAF). The roof system of each EAF is configured accordingly (e.g., a three phase AC EAF would include a roof with three apertures configured to accept three separate electrodes).
In the context of steel production, EAFs melt steel scrap by generating large amounts of heat (e.g., approximately 3,000° F.). The electrodes of an EAF generate such large amounts of heat by arcing between each other as well as to the scrap in the furnace, with oxygen often injected into the melting chamber to aid in heat generation. Accordingly, to be efficient, an EAF must be configured to maintain such high temperatures over a prolonged period of time while simultaneously limiting the amount of heat that escapes.
An EAF's roof is essential to its efficiency as it must be designed to withstand these substantial temperatures over a prolonged period of time. Accordingly, prior art EAF roof systems use large, heavy structures as a means to prevent heat escape and, in turn, allow the EAF to melt steel scrap in an efficient manner. Some of the known, heavy roof structures include a skew and delta. The skew (also known as a water-cooled skew or a delta water ring) is in contact with the furnace and often includes a circuitry of water pipes, which are designed to cool the roof system. More specifically, the prior art water-cooled skews are designed primarily to prevent the skew from melting during the steel making process, and in addition, to cool the delta. However, as discussed below, because of how massive prior art deltas are, the water-cooled skews have little if any cooling effect on the massive prior art deltas. Prior art skews also often include a solid metal round (e.g., cylinder) at the distal end of the skew (i.e., the portion exposed to the melting chamber), which essentially acts as a lightning rod in the event electricity were to arc from the electrodes toward the water pipe circuitry during the steel making process.
The delta is the center piece of the roof system. Deltas are composed of refractory material in order to prevent electricity from arcing between the electrodes and the delta. Refractory material is a non-metallic material that will not conduct electricity from the electrodes and will maintain its physical and chemical properties when exposed to high temperatures. As the center portion of the prior art EAF roof system, the prior art deltas are configured to fit together with the skew. Additionally, prior art deltas are as deep as, if not deeper than, the skew used in prior art systems. Finally, prior art refractory deltas are configured so that the interior surface of the skew is adjacent to the delta during the melting process.
Since the prior art refractory deltas are at least the same depth as the skew, they are also quite heavy, often weighing between 10,000-18,000 pounds. Such large structures are expensive to construct and equally expensive to replace. Additionally, the life span of prior art refractory deltas is relatively short, which, in turn, increases a company's operating costs when using such a roof system. Their short life is primarily attributable to their size as well as the relatively short distance from the molten bath of metal (which is a direct result of prior art deltas being the same depth, if not deeper than, prior art skews). More specifically, the life of a refractory delta is a function of the thermal rating of the refractory material, and continuous exposure to the extreme temperatures required for melting scrap metal eventually causes the refractory material to wear out. This is particularly true when oxygen is used in the melting process, as free oxygen erodes the refractory material in the prior art deltas, especially since the prior art deltas extend the length of the skew and are thus in relatively close proximity with the molten metal in the melting chamber.
The size of prior art deltas not only increases exposure to the heat generated during the melting process (because the bottom of the skew and the bottom of the delta are even), it also makes it very difficult to cool them. The aforementioned water pipes provide some, but limited, cooling of the delta. Also, excess water from water sprayers that are used to cool the electrodes provides some additional cooling (i.e., the electrodes are cooled with continuous streams of water, which splash off the electrodes and on to the delta). However, due to the size of the delta, and how close the bottom of the delta is to the molten metal in the melting chamber, these cooling techniques are inadequate. Thus, the massive deltas need to be replaced more frequently.
An exemplary embodiment of the present disclosure provides a roof system in which the delta is smaller, costs less, and lasts longer than prior art refractory deltas. The present disclosure accomplishes this by employing a new and useful method for making such roof systems. An exemplary method of the present invention includes the steps of creating a first mold and then using the first mold to cast a lining of refractory material to a skew. A second mold is then created and used to cast a delta of refractory material. The refractory delta and refractory-lined skew comprise a part of the roof system of an electric arc furnace.
In another embodiment, an exemplary roof system of the present disclosure includes a skew that is removably attached to an electric arc furnace, the skew having a proximal end, distal end, and an interior surface, a refractory lining that extends from the proximal end of the skew to the distal end thereof, a refractory delta, the delta having a proximal surface, a distal surface and at least one opening capable of receiving at least one electrode, wherein the delta extends distally toward, but not as far as, the distal ends of the skew and refractory lining.
In yet another embodiment, an exemplary roof system comprises a lining of refractory material affixed to the interior surface of a skew of an electric arc furnace, and a delta of refractory material configured to fit on to the lining of refractory material.
And in yet another embodiment, an exemplary roof system comprises a delta of refractory material sized to fit on to the skew. An exemplary method for making this embodiment includes creating a first mold and using the first mold to cast a refractory delta.
Corresponding reference characters indicate corresponding parts throughout the several views.